Methods of generating gas in well treating fluids

ABSTRACT

The present invention relates to methods of generating gas in and foaming well treating fluids during pumping of the treating fluids or after the treating fluids are placed in a subterranean zone, or both. A method of the present invention provides a method of treating a subterranean zone comprising the steps of providing a well treating fluid that comprises a water component, a gas generating chemical, and an encapsulated activator, placing the well treating fluid in a subterranean zone, and allowing the gas generating chemical to react so that a gas is generated in the cement composition. Methods of cementing, fracturing, cementing compositions, fracturing fluid compositions, and foamed well fluid compositions also are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/792,999,entitled “Methods of Generating Gas in Well Treating Fluids,” filed onMar. 4, 2004, which is a continuation of application Ser. No.10/159,588, entitled “Methods of Generating Gas in Well TreatingFluids,” filed on May 31, 2002, both of which are hereby incorporated inentirety by reference.

BACKGROUND

The present invention relates to methods of generating gas in andfoaming well treating fluids during pumping of the treating fluids orafter the treating fluids are placed in a subterranean zone, or both.

Foamed treating fluids have heretofore been utilized in a number of oiland gas well applications. Typically, a treating fluid is foamed bycombining a mixture of foaming and foam stabilizing surfactants with thetreating fluid on the surface followed by injecting gas, typicallynitrogen, into the treating fluid containing the foaming and foamstabilizing surfactants as the treating fluid is pumped to the well headand into the well bore. This process allows the treating fluid to havegas concentrations of 1% to 80% by volume of the fluid depending on thedownhole pressure and temperature and the amount of gas injected atsurface. However, the equipment required for storing the nitrogen inliquid or gaseous form and injecting it into a well treating fluid isvery elaborate and expensive. In addition, the equipment is frequentlyunavailable or cannot be easily transported to well sites due to theirremote locations.

In situ gas forming agents have been utilized heretofore in well cementcompositions to prevent annular gas migration. For example, surfactantcoated finely ground aluminum has been included in cement compositionsto generate hydrogen gas in the compositions as they are being pumpeddown a well bore and after they are placed in the annulus between thewalls of the well bore and casing or other pipe string therein. Thepresence of the gas in the cement compositions prevents formation fluidsfrom entering the cement compositions as the cement compositions developgel strength. That is, the development of gel strength reduces theability of a cement composition column to transmit hydrostatic pressure.If the hydrostatic pressure of the cement composition column falls belowthe formation pore pressure before the cement composition has gainedsufficient strength to prevent the entry of formation fluids into thewell bore, the fluids enter the well bore and form channels in thecement composition column which remain after the cement compositioncolumn sets. The presence of the gas which is generated in the cementcomposition from the finely ground aluminum increases the volume of thecement composition such that the volume increase generated by the gasequals or slightly exceeds the cement composition volume reductionduring the development of gel strength due to fluid loss and/or thecement hydration reaction. The increase in volume and thecompressibility produced in the cement composition by the gas allows thecement composition column to resist the entry of formation fluids intothe well bore.

Other gas forming agents have also been added to well cementcompositions to gasify the compositions. For example, U.S. Pat. No.4,450,010 issued on May 22, 1984 to Burkhalter et al. discloses a wellcementing method and gasified cements useful in carrying out the method.That is, U.S. Pat. No. 4,450,010 discloses a method of cementing insubterranean formations using a gasified cement composition whichprevents formation fluids from entering the cement composition columnformed in the annulus between the well bore and a pipe string therein.The cement composition includes a nitrogen gas generating material, anoxidizing agent, and a reaction rate control material whereby a quantityof gas is generated in the cement composition to offset the shrinkage inthe cement composition column as it develops gel strength and to providecompressibility thereto whereby the entry of formation fluids into thewell bore is reduced or prevented. While the methods and cementcompositions of the prior art have achieved varying degrees ofcommercial success, there are needs for improved methods of generatinggas in well cement compositions and other well fluids whereby the wellfluids can be foamed in situ at various selected times during their useto produce a variety of desired downhole results. The control of thetiming of in situ gas generation until after the well fluids are pumpedalso helps in preventing air-locking of the mechanical pumps used.

A situation where the presence of gas in a cement composition willprovide a distinct advantage involves problems associated with highfluid pressure buildup behind cemented casing. Occasionally, drillingfluid and cement spacer fluids are left behind casing during thecementing of the casing in a well bore. When the well is put onproduction, the formation temperature heats up the trapped drillingand/or spacer fluids causing severe high pressure build ups due to theincompressibility of the fluids which can cause damage to the casing.The presence of a compressible gas behind the casing in drilling fluidand cement spacer fluids, either in the form of a gas pocket or foam,will help sustain the temperature increases without severe pressurebuildups.

SUMMARY

The present invention relates to methods of generating gas in andfoaming well treating fluids during pumping of the treating fluids orafter the treating fluids are placed in a subterranean zone, or both.

In one embodiment, the present invention provides a method of treating asubterranean zone comprising the steps of: providing a well treatingfluid that comprises a water component, a gas generating chemical, andan encapsulated activator; placing the well treating fluid in asubterranean zone; and allowing the gas generating chemical to react sothat a gas is generated in the cement composition.

In another embodiment, the present invention provides a method offracturing a subterranean zone comprising the steps of providing afracturing fluid that comprises a water component, a gelling agent, agas generating chemical, and an encapsulated activator; contacting thesubterranean zone with the fracturing fluid at a pressure sufficient tocreate or enhance at least one fracture therein; allowing the gasgenerating chemical to react so that generated gas is incorporated inthe fracturing fluid; and reducing the viscosity of the fracturing fluidso as to produce a reduced viscosity fracturing fluid.

In another embodiment, the present invention provides a method ofcementing in a subterranean zone comprising the steps of providing acement composition that comprises a hydraulic cement, a water component,a gas generating chemical, and an encapsulated activator; placing thecement composition in a subterranean zone; allowing the gas generatingchemical to react so that a gas is generated in the cement composition;and allowing the cement composition to set in the subterranean zone.

The objects, features and advantages of the present invention will bereadily apparent to those skilled in the art upon a reading of thedescription of preferred embodiments which follows.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides improved methods of generating gas in andfoaming well treating fluids while the treating fluids are being pumpedand/or after being placed in a subterranean zone. In accordance with theinvention, one or more gas generating chemicals and one or more delayedencapsulated activators are combined with a well treating fluid such asa hydraulic cement slurry, a fracturing fluid or the like. The treatingfluid must be alkaline or made alkaline, i.e., the treating fluid musthave a pH in the range of from about 10 to about 14.

The gas generating chemicals useful in accordance with this inventionprimarily generate nitrogen along with small amounts of ammoniadepending on the chemical structure of the gas generating chemical andthe activating agent. When the gas generating chemical molecule containsamide groups, additional ammonia, carbon dioxide (an acidic gas), andcarbon monoxide may be produced. The gas generating chemicals aregenerally solid materials that liberate gas or gases on their own whenthey are heated to a temperature in the range of from about 200° F. toabout 500° F. without requiring alkaline or oxidizing chemicals. Inorder to cause the gas generating chemicals to generate gases atparticular temperatures and/or times, one or more delayed encapsulatedactivators having selected release times are combined with the treatingfluid containing one or more gas generating chemicals. After the gasgenerating chemical or chemicals and delayed encapsulated activator oractivators have been combined with the treating fluid, the treatingfluid is pumped into a subterranean zone to be treated by way of thewell bore.

As mentioned, the gas generating chemicals and delayed encapsulatedactivators can be used to generate gas in and foam a treating fluid atdifferent temperatures and times during pumping and after placement. Forexample, nitrogen gas can be caused to be produced in an aqueous cementcomposition while being pumped to foam the composition and providethixotropy thereto, after being placed in the subterranean zone duringthe static cement composition gel strength development stage tocompensate for cement shrinkage caused by fluid loss, during the cementtransition time to prevent the inflow of formation fluids and duringsetting of the cement to provide resiliency thereto.

Gas generating chemicals which can be utilized in accordance with themethods of the present invention include, but are not limited to,compounds containing hydrazine or azo groups, for example, hydrazine,azodicarbonamide, azobis (isobutyronitrile), p toluene sulfonylhydrazide, p-toluene sulfonyl semicarbazide, carbohydrazide, p-p′ oxybis(benzenesulfonylhydrazide) and mixtures thereof. Additional examples ofnitrogen gas generating chemicals which do not contain hydrazine or azogroups and which are also useful in the present invention include, butare not limited to, ammonium salts of organic or inorganic acids,hydroxylamine sulfate, carbamide and mixtures thereof. Of these,azodicarbonamide or carbohydrazide are preferred. The gas generatingchemical or chemicals utilized are combined with the well treating fluidin a general amount, depending on the amount of gas desired underdownhole conditions, in the range of from about 0.1% to about 10% byweight of the treating fluid, more preferably in an amount in the rangeof from about 0.3% to about 8% and most preferably about 4%.

The generation of gas from the gas generating chemicals depends on thestructure of the gas generating chemicals. When the chemical contains anazo group containing two nitrogens connected by a double bond as inazodicarbonamide, the gas generation is caused either thermally or byreaction with alkaline reagents. The reactions with the azocarbonamidegenerate ammonia gas and possibly carbon dioxide and release the doublycharged diimide group. The diimide dianion being chemically unstabledecomposes to nitrogen gas.

The gas generating chemicals containing hydrazide groups in which thetwo nitrogen atoms are connected by a single bond as well as connectedto one or two hydrogens produce gas upon reaction with an oxidizingagent. It is believed that the oxidizing agent oxidizes the hydrazidegroup to azo structure. Therefore, hydrazide materials containing twomutually single bonded nitrogens which in turn are also bonded to one ormore hydrogens need oxidizing agents for activation. To enhance thewater solubility of such materials, alkaline pH is generally required.Occasionally, additional chemicals may be needed to increase the rate ofgas production.

Examples of delayed encapsulated activators that can be used include,but are not limited to, alkaline materials such as carbonate, hydroxideand oxide salts of alkali and alkaline earth metals such as lithium,sodium, magnesium and calcium and oxidizing agents such as alkali andalkaline earth metal salts of peroxide, persulfate, perborate,hypochlorite, hypobromite, chlorite, chlorate, iodate, bromate,chloroaurate, arsenate, antimonite and molybate anions. Specificexamples of the oxidizing agents include ammonium persulfate, sodiumpersulfate, potassium persulfate, sodium chlorite, sodium chlorate,hydrogen peroxide, sodium perborate and sodium peroxy carbonate. Otherexamples of oxidizers which can be used in the present invention aredisclosed in U.S. Pat. No. 5,962,808 issued to Landstrom on Oct. 5, 1999which is incorporated herein by reference thereto. Of the variousactivators that can be used, sodium or ammonium persulfate and sodiumchlorite are preferred. The active amounts of the oxidizing activator oractivators included in the well treating fluid in the encapsulated formrange from about 2 to about 15 times the weight of the gas generatingchemical or chemicals therein, more preferably in an amount of about 4to about 12 times the weight of the gas generating chemical. The actualamounts of the alkaline material used in the well treating fluid shouldbe sufficient to maintain the pH of the fluid between 10 and 14. Theactivator or activators used and their amounts are selected for theactivator's ability to cause the gas generating chemical or chemicals togenerate gas at a particular temperature or range of temperatures. Thetemperatures at which various activators cause a particular gasgenerating chemical to produce gas can be readily determined in thelaboratory.

The activators can be encapsulated with various materials which delaytheir reaction with the gas generating chemical or chemicals used. Solidactivators can be encapsulated by spray coating a variety of materialsthereon. Such coating materials include, but are not limited to, waxes,drying oils such as tung oil and linseed oil, polyurethanes andcross-linked partially hydrolyzed polyacrylics. Of these, cross-linkedpartially hydrolyzed acrylics are preferred. Because of the oxidizingand corrosive nature of the activators, an additional undercoat ofpolymeric materials such as styrene butadiene may be deposited on thesolid activator particles prior to depositing the slow releasingpolymeric coating. This method is particularly suitable forencapsulating hygroscopic alkaline activator materials such as hydroxidesalts of lithium, sodium and potassium and is described in detail inco-pending U.S. patent application Ser. No. 09/565,092 filed on May 5,2000 entitled Encapsulated Chemicals For Use In Controlled Time ReleaseApplications And Methods. The oxidizers are preferably encapsulated witha membrane comprising a partially hydrolyzed acrylic cross-linked witheither an aziridine prepolymer or a carbodiimide, the membrane havingimperfections through which an aqueous fluid can diffuse. Thecross-linked acrylic membrane and its use are disclosed in detail inU.S. Pat. No. 5,373,901 issued to Norman et al. on Dec. 20, 1994 whichis incorporated herein by reference thereto. The activators may also beencapsulated in the form of aqueous solutions in a particulate poroussolid material which remains dry and free flowing after absorbing anaqueous solution and through which the aqueous solution slowly diffuses.Examples of such particulate porous solid materials include, but are notlimited to, diatomaceous earth, zeolites, silica, alumina, metal saltsof alumino-silicates, clays, hydrotalcite, styrene-divinylbenzene basedmaterials, cross-linked polyalkylacrylate esters and cross-linkedmodified starches. Of these, metal oxides, metal salts ofalumino-silicates and cross-linked porous synthetic polymeric materialsare preferred with precipitated silica being the most preferred. Whenthe activators are alkaline materials, the inorganic porous carrierssuch as porous silica, alumina, or diatomaceous earth are not preferablesince they react with the alkaline materials.

In order to provide additional delay to the oxidizing agent activatorsencapsulated in a particulate porous solid material described above, anexternal coating of a polymeric material through which an aqueoussolution slowly diffuses can be placed on the porous solid material.Examples of such polymeric materials include, but are not limited to,EDPM rubber, polyvinyldichloride (PVDC), nylon, waxes, polyurethanes andcross linked partially hydrolyzed acrylics. Of these, cross-linkedpartially hydrolyzed acrylics are preferred. The particulate poroussolid materials and their use for encapsulating activators and the likeare disclosed in detail in U.S. Pat. No. 6,209,646 BI issued on Apr. 3,2001 which is incorporated herein by reference thereto.

A gas production rate enhancing chemical may be used when rapid gasproduction is desired. Examples of such rate enhancing chemicals whichcan optionally be used include, but are not limited to, copper saltssuch as copper sulfate, ethylene diamine tetraacetic acid (EDTA)complexes of copper (2+) salts, iron salts including ferric sulfate orferric nitrate. When the gas generation from the mixture of a gasgenerating chemical and an activator does not take place unless the rateenhancing material is present due to low application temperature or thelike, the timing of the production of gas can be controlled by usingencapsulated rate enhancing materials. The encapsulation methods used toencapsulate the rate enhancing materials are the same as those describedabove for encapsulating the activator materials.

In addition to the gas generating chemical or chemicals and delayedencapsulated activator or activators, a mixture of foaming and foamstabilizing surfactants can be combined with the treating fluid tofacilitate the formation and stabilization of the treating fluid foamproduced by the liberation of gas therein. An example of such a mixtureof foaming and foam stabilizing surfactants which is preferred for usein accordance with this invention is comprised of an ethoxylated alcoholether sulfate surfactant, an alkyl or alkene amidopropylbetainesurfactant and an alkyl or alkene amidopropyldimethylamine oxidesurfactant. Such a surfactant mixture is described in U.S. Pat. No.6,063,738 issued to Chatterji et al. on May 16, 2000 which isincorporated herein by reference thereto.

When the treating fluid in which gas is to be generated in accordancewith this invention is an alkaline well cement composition, one or moregas generating chemicals as described above are included in the cementcomposition. Preferably, the gas generating chemicals are selected fromthe group consisting of azodicarbonamide, carbohydrazide and mixturesthereof. One or more delayed encapsulated activators having selectedrelease times are combined with the cement composition containing thegas generating chemical or chemicals so that the gas generating chemicalor chemicals react with one or more delayed encapsulated activatorswhile the cement composition is being pumped and at one or more timesafter the cement composition has been placed in the subterranean zone tobe cemented. After the gas generating chemical or chemicals and delayedencapsulated activator or activators have been combined with the cementcomposition, the cement composition is pumped into the well bore andinto the subterranean zone to be cemented.

The quantity of gas generating chemical or chemicals combined with thecement composition and the number of delayed encapsulated activatorshaving different release times can be selected and included in thecement composition so that gas is formed in the cement compositionduring one or more of the following stages. During pumping to foam thecement composition and provide thixotropy thereto, after the cementcomposition is placed in the subterranean zone to be cemented during thestatic cement composition gel strength development stage to compensatefor cement shrinkage due to fluid loss or the like, during the cementcomposition transition time to prevent the inflow of formation fluid andduring the setting of the cement to provide resiliency thereto. The term“cement composition transition time” is used herein to mean the timefrom when the cement composition column increases in gel strength to thelevel whereby there is a loss of ability to transmit hydrostaticpressure to when the cement composition sets into a hard impermeablemass.

The hydraulic cement compositions which can be utilized in accordancewith the methods of this invention are basically comprised of ahydraulic cement, water present in an amount sufficient to form aslurry, the above described gas generating chemical or chemicals and theabove described delayed encapsulated oxidizing agent activator oractivators.

A variety of hydraulic cements can be utilized in the cementcompositions including those comprised of calcium, aluminum, silicon,oxygen and/or sulfur which set and harden by reaction with water. Suchhydraulic cements include Portland cements, pozzolana cements, gypsumcements, aluminous cements and silica cements. Portland cements or theirequivalents are generally preferred for use in accordance with thepresent invention. Portland cements of the types defined and describedin the API Specification For Materials And Testing For Well Cements, APISpecification 10, 5th edition, dated Jul. 1, 1990 of the AmericanPetroleum Institute are particularly suitable. Preferred API Portlandcements include classes A, B, C, G and H, with API classes G and H beingthe most preferred.

The water utilized in the cement compositions can be fresh water,unsaturated aqueous salt solutions such as brine or seawater andsaturated aqueous salt solutions. The water is generally present in thecement compositions in an amount sufficient to form a slurry, i.e., anamount in the range of from about 30% to about 100% by weight ofhydraulic cement in the compositions, more preferably in an amount inthe range of from about 35% to about 60%.

As is well understood by those skilled in the art, the cementcompositions of this invention can include a variety of additives forimproving or changing the properties of the cement compositions.Examples of such additives include, but are not limited to, setretarding agents, fluid loss control agents, dispersing agents, setaccelerating agents and formation conditioning agents.

As mentioned above, another treating fluid which can be utilized inaccordance with the methods of this invention is a fracturing fluid forcreating, extending and propping fractures in a subterranean zone tostimulate the production of hydrocarbons therefrom. The fracturing fluidutilized is generally a viscous alkaline fracturing fluid which formsthe fractures in the subterranean zone and deposits proppant therein.Thereafter, the fracturing fluid breaks into a thin fluid which isproduced back to the surface. Gas is generated in the fracturing fluidto facilitate the back flow of the fracturing fluid and its removal fromthe fractures. In accordance with the methods of this invention, one ormore gas generating chemicals and one or more delayed encapsulatedactivators having selected release times are combined with thefracturing fluid on the surface. Thereafter, the fracturing fluid ispumped into the well bore and into a subterranean zone whereby fracturesare formed in the zone. After the formation of the fractures, thefracturing fluid which includes a viscosity breaker reverts to a thinfluid, the pressure exerted on the fractured zone is reduced and gas isgenerated by the activated gas generating chemical or chemicals therein.The presence of the gas facilitates the back flow of the fracturingfluid from the fractures and its removal from the subterranean zone.

While a variety of fracturing fluids can be utilized, a preferredfracturing fluid for use in accordance with this invention is basicallycomprised of water, a hydrated galactomannan gelling agent, a retardedcross-linking and buffering composition which cross-links the hydratedgalactomannan gelling agent and produces a highly viscous alkalinefluid, a delayed gel breaker for causing the viscous fracturing fluid tobreak into a thin fluid, one or more of the gas generating chemicalsdescribed above and one or more of the delayed encapsulated activatorsdescribed above.

The water utilized for forming the fracturing fluid can be fresh water,salt water, brine or any other aqueous liquid which does not adverselyreact with other components of the fracturing fluid.

The galactomannan gelling agents which can be used are the naturallyoccurring gums and their derivatives such as guar, locust bean, tara,honey locust, tamarind, karaya, tragacanth, carragenan and the like. Ofthe various galactomannan gelling agents which can be utilized, one ormore gelling agents selected from the group of guar and guar derivativesare preferred. Examples of guar derivatives which can be used includehydroxyethylguar, hydroxypropylguar, carboxymethylguar,carboxymethylhydroxyethylguar and carboxymethylhydroxypropylguar. Ofthese, carboxymethylhydroxypropylguar is the most preferred.

While various cross-linking agents or compositions can be utilized, aretarded cross-linking composition comprised of a liquid solvent, anorganotitanate chelate and a borate ion producing compound is generallypreferred. Various delayed gel breakers can also be utilized in thefracturing fluids of this invention. A preferred delayed breaker is amixture of calcium hypochlorite or an alkali metal chlorite orhypochlorite and optionally, an activator such as a copper ion producingcompound, e.g., copper EDT A. Such breakers and activators are describedin U.S. Pat. No. 5,413,178 issued to Walker et al. on May 9, 1995; U.S.Pat. No. 5,669,446 issued to Walker et al. on Sep. 23, 1997; and U.S.Pat. No. 5,950,731 issued to Suchart et al. on Sep. 14, 1999, thedisclosures of which are all incorporated herein by reference thereto.

Thus, an improved method of generating gas in and foaming an alkalinewell treating fluid introduced into a subterranean zone penetrated by awell bore is comprised of the following steps: (a) combining one or moregas generating chemicals with a well treating fluid; (b) combining oneor more delayed encapsulated activators having selected release timeswith the well treating fluid containing the gas generating chemicalsformed in step (a) so that the one or more gas generating chemicalsreact with the one or more delayed encapsulated activators and gas andfoam are formed in the treating fluid while the treating fluid is beingpumped or at one or more times after the treating fluid has been placedin the subterranean zone, or both; and (c) pumping the treating fluidformed in step (b) into the well bore and into the subterranean zone.

An improved method of this invention for generating gas in an alkalinewell cement composition introduced into a subterranean zone penetratedby a well bore is comprised of the following steps: (a) combining one ormore gas generating chemicals selected from the group consisting ofazodicarbonamide, carbohydrazide and mixtures thereof with the cementcomposition; (b) combining one or more delayed encapsulated oxidizingagent activators having selected release times with the cementcomposition containing the gas generating chemicals formed in step (a)so that the gas generating chemicals react with the one or more delayedencapsulated oxidizing agent activators and gas is formed in the cementcomposition while the cement composition is being pumped and at one ormore times after the cement composition has been placed in thesubterranean zone, the oxidizing agents in the delayed encapsulatedoxidizing agent activators having selected release times being selectedfrom the group consisting of alkali and alkaline earth metal salts ofperoxide, persulfate, perborate, hypochlorite, hypobromite, chlorate,iodate, bromate, chloroaurate, arsenate, antimonite and molybate 16anions and (c) pumping the cement composition formed in step (b) intothe well bore and into the subterranean zone.

An improved method of generating gas in a viscous alkaline wellfracturing fluid which is introduced into a subterranean zone penetratedby a well bore, which forms fractures in the subterranean zone and thenbreaks into a thin fluid comprises the steps of: (a) combining one ormore gas generating chemicals with the fracturing fluid; (b) combiningone or more delayed encapsulated activators having selected releasetimes with the fracturing fluid containing the gas generating chemicalsformed in step (a) so that the gas generating chemicals react with thedelayed encapsulated activators and gas is formed in the fracturingfluid after fractures have been formed in the subterranean zone andduring and after the fracturing fluid breaks into a thin fluid wherebythe gas facilitates the back flow of the fracturing fluid and itsremoval from the fractures; and (c) pumping the fracturing fluid formedin step (b) into the well bore and into the subterranean zone.

In order to further illustrate the methods of the present invention, thefollowing example is given. This example should not be used improperlyto limit or define the invention.

EXAMPLE

A cement slurry was prepared by mixing 748 grams of API Class G cement,336 grams of deionized water, 7.5 grams of bentonite of clay, 1.5 gramsof carboxymethylhydroxyethyl cellulose, 3.0 grams lignosulfonateretarder and 36.3 grams of ammonium persulfate in a Waring blenderaccording to API Specifications. A foaming and foam stabilizingsurfactant was hand mixed into the slurry followed by 3.0 grams oftoluenesulfonyl hydrazide. The calculated density of the slurry was15.80 pounds per gallon. The gas evolution with concurrent foaming ofthe slurry was instantaneous. The slurry was allowed to expand for 420minutes. As shown in Table I below, the slurry density measured at theend of this period was found to be 3.39 pounds per gallon. The percentof nitrogen formed was calculated to be 78.3% by volume of the slurry.

In a second experiment, 45.4 grams of encapsulated ammonium persulfatewith 80% active content were used in place of the non-encapsulatedammonium persulfate used in the first experiment. 100 cc of the cementslurry described in the first experiment except for the change describedherein were added to a graduated cylinder and the volume increase of theslurry was measured over time. This experiment was conducted asdescribed in the first experiment. The results are also presented inTable 1 below.

In a third experiment, a sample of encapsulated ammonium persulfate(59.5 grams) with 60% active content was used in place of thenon-encapsulated ammonium persulfate used in the first experiment. Theencapsulated material used in this experiment had a higher polymercoating and was designed to release the encapsulated material moreslowly than that used in the second experiment. This experiment was alsoconducted as described in the first experiment. The volume increase ofthe slurry over time is also presented in Table 1 below.

In a fourth experiment, the toluenesulfonyl hydrazide used in the firstexperiment was replaced by 5.6 grams of carbohydrazide and the amount ofnon-encapsulated ammonium persulfate was decreased to 28 grams. Thisexperiment was also conducted as described in the first experiment. Thegas evolution was again found to be instantaneous. As shown in Table 2,the set cement density at the end of 18 hrs. was 3.89 pounds per gallon,which corresponded to the presence of 75% nitrogen gas.

In a fifth experiment, the non-encapsulated ammonium persulfate used inthe fourth experiment was replaced by 34.8 grams encapsulated ammoniumpersulfate with 80% active component. This experiment was also conductedas described in the first experiment. The cement density measured at theend of 18 hrs. was 14.12 pounds per gallon. The volume increase of theslurry over time is presented in Table 2 below.

In a sixth experiment, the non-encapsulated ammonium persulfate used inthe fourth experiment was replaced by 45.8 grams of encapsulatedammonium persulfate with 60% active component. This experiment was alsoconducted as described in the first experiment. The final density at theend of 18 hrs. was 14.62 pounds per gallons. The volume increase of theslurry over time is given in Table 2 below. TABLE 1 Ammonium PersulfateEncapsulated Encapsulated (non- Ammonium Ammonium encapsulated),Persulfate I¹, Persulfate II², Time 5.0% by weight 6.25% by 8.33% by Inof cement weight of cement weight of Minutes Volume Volume cement Volume0 Instantaneous³ 100 ml 100 ml 5 ″ 105 ml 100 ml 10 ″ 106 ml 100 ml 15 ″108 ml 100 ml 30 ″ 130 ml 100 ml 60 ″ 138 ml 101 ml 90 ″ 176 ml 104 ml120 ″ 176 ml 106 ml 150 ″ 176 ml 110 ml 180 ″ 176 ml 114 ml 210 ″ 176 ml128 ml 240 ″ 176 ml 130 ml 300 ″ 176 ml 140 ml 360 ″ 176 ml 148 ml 420 ″176 ml 150 ml Slurry Density @ 3.39 lb/gal (after 11.49 lb/gal 11.9lb/gal 420 minutes foaming)¹80% active ammonium persulfate content²60% active ammonium persulfate content³The gas evolution was complete in less than a minute

TABLE 2 Ammonium Encapsulated Persulfate Encapsulated Ammonium Time(non-encapsulated), Ammonium Persulfate In 3.75% Persulfate I¹, 4.69%II², 8.33% Minutes by weight of cement by weight of cement by weight ofcement Volume Volume Volume  0 Instantaneous³ 100 ml 100 ml  5 ″ 102 ml100 ml 10 ″ 104 ml 100 ml 15 ″ 106 ml 100 ml 30 ″ 120 ml 100 ml 60 ″ 166ml 100 ml 90 ″ 180 ml 102 ml 120  ″ 180 ml 102 ml 18 hrs. ″ 180 ml 160ml Ulti- 3.89 lb/gal 14.12 lb/gal (base) 14.62 lb/gal mate (afterfoaming) Density @ 18 hrs.¹80% active ammonium persulfate content²60% active ammonium persulfate content³The gas evolution was complete in less than 1 minute

The results in Tables 1 and 2 show that the nitrogen gas generation canbe delayed by controlling the release of the oxidizing agent into thecement slurry. The desired rate of gas generation can be accomplished bycontrolling the amount of the encapsulating coating.

Thus, the present invention is well adapted to carry out the objects andattain the ends and advantages mentioned as well as those which areinherent therein. While numerous changes can be made by those skilled inthe art, such changes are encompassed within the spirit of thisinvention as defined by the appended claims.

1. A method of treating a subterranean zone comprising the steps of:providing a well treating fluid that comprises a water component, a gasgenerating chemical, and an encapsulated activator; placing the welltreating fluid in a subterranean zone; and allowing the gas generatingchemical to react so that a gas is generated in the cement composition.2. The method of claim 1 wherein the gas generating chemical comprises ahydrazine or an azo group.
 3. The method of claim 1 wherein the gasgenerating chemical is present in the well treating fluid in an amountin the range of from about 0.1% to about 10% by weight of the welltreating fluid.
 4. The method of claim 1 wherein the encapsulatedactivator comprises an alkaline material or an oxidizing agent.
 5. Themethod of claim 1 wherein the encapsulated activator comprises at leastone of the following: a coating material, an undercoating, or a membranethat has at least one imperfection through which an aqueous fluid maydiffuse.
 6. The method of claim 5 wherein the coating material comprisesa wax, a drying oil, a polyurethane, a polyacrylic, or styrenebutadiene.
 7. The method of claim 5 wherein the undercoating comprisesstyrene butadiene.
 8. The method of claim 5 wherein the membranecomprises a partially hydrolyzed acrylic cross-linked with an aziridineprepolymer or a carbodiimide.
 9. The method of claim 1 wherein at leasta portion of the encapsulated activator is in the form of an aqueoussolution on porous solid particulates.
 10. The method of claim 9 whereinthe portion of the encapsulated activator that is in the form of anaqueous solution on porous solid particulates further comprises apolymeric external coating.
 11. The method of claim 1 wherein the welltreating fluid further comprises a gas production rate enhancingchemical.
 12. The method of claim 11 wherein the gas production rateenhancing chemical comprises a copper salt, a complex of a copper salt,an iron salt, or ethylene diamine tetraacetic acid.
 13. A method offracturing a subterranean zone comprising the steps of: providing afracturing fluid that comprises a water component, a gelling agent, agas generating chemical, and an encapsulated activator; contacting thesubterranean zone with the fracturing fluid at a pressure sufficient tocreate or enhance at least one fracture therein; and allowing the gasgenerating chemical to react so that generated gas is incorporated inthe fracturing fluid; and reducing the viscosity of the fracturing fluidso as to produce a reduced viscosity fracturing fluid.
 14. The method ofclaim 13 wherein the gelling agent comprises a cross-linked gellingagent.
 15. The method of claim 13 wherein the fracturing fluid furthercomprises a crosslinking agent.
 16. The method of claim 13 wherein thefracturing fluid further comprises a delayed gel breaker.
 17. The methodof claim 13 wherein the encapsulated activator comprises at least one ofthe following: a coating material, an undercoating, or a membrane thathas at least one imperfection through which an aqueous fluid maydiffuse.
 18. A method of cementing in a subterranean zone comprising thesteps of: providing a cement composition that comprises a hydrauliccement, a water component, a gas generating chemical, and anencapsulated activator; placing the cement composition in a subterraneanzone; allowing the gas generating chemical to react so that a gas isgenerated in the cement composition; and allowing the cement compositionto set in the subterranean zone.
 19. The method of claim 18 wherein thehydraulic cement comprises a Portland cement, a pozzolana cement, agypsum cement, an aluminous cement, or a silica cement.
 20. The methodof claim 18 wherein the encapsulated activator comprises at least one ofthe following: a coating material, an undercoating, or a membrane thathas at least one imperfection through which an aqueous fluid maydiffuse.